| While there is great potential immunotherapies have at best only resulted in modest increases in survival; with only a small fraction of patients being cured. Of many contributors to the failure of immunotherapies, immunosuppression conferred by the PD-1/PD-L1 axis appears to be a prevalent mechanism, based on a number of studies done over the past decade. Although the signaling and downstream immunosuppressive effects of the PD-1/PD-L1 axis are well-studied in detail in T and B cells, more recent evidence of PD-1 expression on tumor-infiltrating innate cells, such as tumor-infiltrating DCs (TIDCs), shed additional light on the complexity of this network in regulating the immune response. TIDCs can comprise of up to 40% of infiltrating immune cells in many tumor types including ovarian cancer and are often associated with worse outcomes. With progression of disease, these cells gradually lose their immunostimulatory potential. Of many potential contributors to such phenotypic and functional changes, this may include the progressive tumor microenvironment (TME)-induced expression of PD-1 on TIDCs. PD-1 expression is local to the TME as the peripheral DCs in tumor-bearing patient do not increase PD-1 expression; suggesting a role of TME factors in induction of PD-1 expression on TIDCs. Our results show that IL-10 is capable of inducing PD-1 expression on the DCs.;We have found that IL-10, expressed at high levels in ascites of ovarian cancers in both humans and mice increases PD-1 expression on TIDCs. Along with that induction, IL-10 also increases the expression of PD-L1, both soluble and cell-surface bound forms as well, hence contributing profound immune dysfunction. Moreover, we show that the IL-10-mediated increase in PD-1 expression depends on STAT3 activity. I further show that blockade of PD-1 on TIDCs with PD-1 blocking antibodies, leads to an increased release of IL-10 by these cells, which in turn is capable of further inducing PD-1 expression on DCs, thereby creating an pathologic immune suppressive loop.;Additionally, we have demonstrated that PD-1 expressed on TIDCs inhibits their ability to respond to danger signals via down modulation of inflammatory cytokines, co-stimulatory molecules expression. We show that PD-1 expressed on DCs paralyzes them by inhibiting production of cytokines, co-stimulatory molecules expression, and antigen presentation by inhibition of NFkappaB activation. These effects were also both dependent (cytokine production) and independent (co-stimulatory molecules expression and antigen presentation) of SHP-2 activity.;Finally, I show that combination treatment of PD-1 blockade and IL-10 signaling antagonism, using blocking antibodies, significantly increases the survival of tumor bearing mice, when compared to either treatment alone. Combination treatment significantly reduces the tumor burden in these mice and augments anti-tumor responses as evidenced by increased infiltration by activated/mature immune cells as well as enhanced anti-tumor antibody and T-cell responses. This was accompanied by decrease in suppressive cells such as MDSCs.;Our findings elucidate a mechanism on how factors/cytokines from TME can induce PD-1 expression on the infiltrating immune cells. Although, blockade of PD-1 has shown promising results in the clinic for patients with different tumors, the response is often limited and suboptimal. As evidenced by our in vivo data, we show that in tumors that do not respond well to PD-1 monotherapy, combination anti-IL-10 and anti-PD-1 shows promising results. These results open up avenues to explore combination therapies to target tumors that employ multiple compensatory inhibitory circuits to inhibit effective anti-tumor immune responses. |
